1. Field of the Invention
The present invention relates to expansion turbine having a variable nozzle mechanism used in large refrigerators such as helium refrigerator. Priority is claimed on Japanese Patent Application No. 2007-089478, filed Mar. 29, 2007, the content of which is incorporated herein by reference.
2. Description of Related Art
Expansion turbines have been used conventionally to enhance the efficiency of refrigerators. To regulate the flow rate of gas introduced into such an expansion turbine, as shown in FIG. 5, expansion turbines using variable nozzle mechanism 10 are popularly used (for example, refer to the Japanese Unexamined Patent Application, First Publication No. 2001-132410.)
This variable nozzle mechanism 10 comprises a nozzle member 14 used to change the throat area of very low temperature gas introduced into a turbine impeller 12, and a driving member 16 used to drive the nozzle member 14. The nozzle member 14 is built into an adiabatic expansion device 20 located in a vacuum container 18. The driving member 16 is disposed outside the vacuum container 18 so as to not expose it to low temperatures and thereby ensure mechanical reliability.
As shown in FIG. 5, the nozzle member 14 and the driving member 16 are connected to each other by a thin cylindrical member 22 coaxial with a turbine impeller 12.
The nozzle member 14 is driven by the oscillation of the cylindrical member 22 around the axial center C of the turbine impeller 12. The nozzle member 14 is disposed to surround the turbine impeller 12. It includes a plurality of movable nozzle plates 14a each oscillatably supported by a support pin 24, and a drive disc 28 connected to the inside end of a cylindrical member 22 and to each of the movable nozzle plate 14a by a drive pin 26. These are pressed against the adiabatic expansion device 20 after receiving a biasing force in the direction of the axial center C by a retaining spring 30.
Moreover, the driving member 16 comprises a rotating drive device 36 such as a pulse motor for driving an oscillatable gear 32 through a gear 34 with the center as the axial center C of the turbine impeller 12 connected to the outside end of the cylindrical member 22.
This variable nozzle mechanism 10 oscillates the cylindrical member 22 about the axial center C of the turbine impeller 12 by driving the rotating drive device 36, oscillates the drive disc 28, and oscillatably drives the movable nozzle plate 14a about the support pin 24 taken as the center, and changes the angle of the movable nozzle plate 14a. In this manner, by continuously changing the throat area of the variable nozzle, the flow rate of gas passing through is regulated.
In expansion turbines using such a conventional variable nozzle mechanism 10, the driving member 16, the cylindrical member 22, the gear 32 and the drive unit 40 including the rotor shaft 38 are configured to be removed as an integral body from the adiabatic expansion device 20 in the vacuum container 18.
However, in the variable nozzle mechanism 10 of the conventional expansion turbine mentioned above, when the drive unit 40 is removed from the adiabatic expansion device 20 in the vacuum container 18 during maintenance, the turbine impeller 12 and the nozzle member 14 tend to be left behind in the vacuum container 18 on the low temperature side. For this reason, when the nozzle member 14 is removed for inspection of coating condition of nozzle or replacement of parts, it becomes necessary to break the vacuum within the vacuum container 18 once, and then remove the nozzle member 14 from the low temperature piping, thus necessitating disassembly work on the low temperature piping side of the vacuum container 18.
In this way, it was very troublesome and required much efforts to remove the nozzle member 14 from the variable nozzle mechanism 10 of the conventional expansion turbine; thus, maintenance work such as replacement and inspect ion of the nozzle member 14 was not easy. Particularly, when degradation or wear exists in the nozzle member 14, for instance, when the flow passage surface of the nozzle peels out or wears out, leak of gas passing through the throat is induced, which has an adverse effect on the performance of the expansion turbine.
For this reason, the development of a variable nozzle mechanism for which maintenance can be carried out easily and which has no adverse effect on the original performance of the expansion turbine, was demanded.
After considering the circumstances mentioned above, the present invention has the object of offering an expansion turbine having a variable nozzle mechanism that enables the nozzle member to be removed easily together with the drive unit, permits maintenance to be performed easily, and moreover has no adverse effect on the original performance of the expansion turbine.